Actually, the side of his abdomen, not his heart.

I don't think they usually did this as bodies were left to hang till they rotted.

In the case of Jesus, his body was being guarded because there was such a crowd. His execution was a political one, he had committed no crime. The Jews at the time were always teetering on the edge of a rebellion and he had already been recognized by many of his followers as the Messiah, which had a particular lore to it.

That's also why the tomb was being guarded. Rome didn't usually guard tombs either. They wanted to make sure the body didn't disappear so one group couldn't claim a miracle to whip up an insurrection.

It's all part of the miracle of the resurrection. In fact, had his execution and entombment not been so closely guarded, Christianity would have never happened because nobody would have believed the resurrection happened. It would have been assumed that someone took his body.